Liquids have been rendered into droplets by a variety of means but most commonly by shearing a liquid stream. The shearing may be introduced by several methods and the particle size distribution of the resulting atomized droplets may be controlled dependent upon several factors based upon the method used. The simplest method to introduce shear is by forcefully ejecting the liquid through a constriction of a desired shape to cause increased perturbations on the liquid stream. Break up devices may be inserted in the path of the stream to introduce secondary shear. Further shear is introduced by drag of the atmopshere through which the stream passes, much as experienced, for instance by a free falling liquid, known as atmospheric drag shear. Shear may also be introduced by vibratory means. Liquid films may be sheared as filaments leave a spinning disc or cup. Additional shear may be introduced by intersecting the liquid stream with a second fluid stream, either gas or liquid. The two most common methods are variations of the first and last methods.
Applications of these techniques range from spraying water, to paints, to applying insecticides, to medicines, and include forming metal powders for special metallurgical applications. Many applications do not warrant attempts at improvement since energy requirements and complications detract from the present simplicity of the process with no additional advantages. Many processes can be improved, however, where a uniform droplet size distribution is required in a specific size range. As may be expected, the smaller the size, the more difficult this is to achieve. Many processes can be improved or simplified where droplet production is required in harsh environments or in the use of hazardous materials. Particular efforts have been made in improving gas to liquid coupling in two diverse fluid systems by configurational modifications of the spraying apparatus and by increasing the energy of the gas. In addition, the particle size distribution may be controlled by sonic and ultrasonic vibrations imposed upon the gas stream; some of these aproaches are described in U.S. Pat. Nos. 2,997,245, 3,067,956, 3,829,301, and 3,909,921. In general, particle size distribution directly relatable to gas velocities or vibrational frequencies has not been demonstrated, since particle size distribution for these designs of the prior art related directly to total gas flow only. The sonic velocities of a two-phase flow when a gas stream couples with a liquid stream were not considered. While very small particle sizes have been possible in the prior art, the sizes obtained were more related to the increased gas pressure than the imposed frequency of a second stream.